Your question is answered sufficiently i am assured! However there are two points, which one "Rich" pointed out in his comment above (nicely). The other would be, to give your customers once you installed Ubuntu/Linux on their boxes a short instruction, since the switch between systems is sometimes a difficult process and point them towards some good sources, such as this page, to make things easier for them. This way their learning curve improves quickly, and they'll see they advantages of both support from and to the Open Source Community. "GL and happy customers!"
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v2rMar 22 '12 at 18:24

Not only is it PERFECTLY LEGAL to install Ubuntu oon anyone's computer at all, IT IS ENCOURAGED AND DELIGHTEDLY SO!

It would behoove you to inform your customer(s) prior to doing this. However, the Customer WINS:

They NEVER WILL NEED TO PAY THE ANNUAL "MICROSOFT TAX" AGAIN (which amounts to HUNDREDS-OF-DOLLARS)!

They will never again have some obscure corporate entity claim it "OWNS YOUR DATA"!

They are free to install this same FREE ("as in "FREE-BEER") software on ANY computer they choose - FOREVER!

Operating System Version Upgrades are ALWAYS FREE!

The Linux Commmunity FIXES THINGS THAT BREAK WAY FASTER AND MORE EFFECTIVELY THAN MICRO-&-SOFT, and Linux doesn't charge you for those fix/upgrades - EVER!

There is a stupifying number of free websites and support forums to get help at every stage of your learning Linux.

There are thousands of videos available on YouTube and other sites that will SHOW and teach you how to do almost anything on Linux; Ubuntu even has VideoCasts available for these purposes - and you're going to LOVE the price -- $0.00 [FREE]!

If you want career-oriented professional training or to learn toward software certification (same thing as geting a Microsoft Certified Support Certification), most commercial IT training and learning centers are fully prepared to sell you training courses to assist you that way.

There are many more reasons to Drop-Kick Bill-the-Gates into the Northern Pacific Ocean and GO-LINUX today, but this ought to give you and your lucky customers a start. Rest assured, your customers will REALLY thank you for this!